1. Field of the Invention
The present invention relates to a filter nozzle with annular filter discs that is provided with flow passages which are radial and which also penetrate the filter discs to serve for conducting the filter liquid.
2. DESCRIPTION OF THE PRIOR ART
For serving the public with drinking and industrial water as well as for the purification of waste water, ever increasing numbers of water treatment systems are being installed. Their purpose is the separation of suspended matter from raw water. These treatment systems--so-called fast filters--mostly consist of one or more open filtering basins or of tanks in closed filters. The filter nozzles are built into the false bottom. On the one hand these filter nozzles have the function during the filtering process of holding back the filter medium and allowing the water to flow through. On the other hand, during the backwashing process the filter medium must be cleaned by means of an opposite stream of rinse water and often of scavenging air. Normally, fine-grain sand serves as the filter medium.
For this purpose the false bottom is furnished with a large number of these filter nozzles. These make possible a good distribution across the entire filter zone of the water to be filtered or of the rinse water or scavenging air. In themselves, these filter nozzles have already been known for many years. They are built in the most diverse ways and more or less satisfy their purposes. In most recent times, the composition of the filter materials has often been changed with respect to quality and/or grain coarseness.
As a consequence of this, the reduced raw water qualities and the higher demands on the filter must be compensated for with higher backwashing parameters, particularly increased rinse water velocities. Connected with this are difficulties with the conventional filter nozzles, which are not satisfactory for these new, increased requirements.
A known phenomenon is the blocking or sedimentation of the filter nozzle due to lodgement of foreign bodies or deposits within the filter nozzle itself. This results in a reduction of the through-flow of water, but also a deterioration of the backwashing process by reason of higher pressure drop in the filter nozzle--and by this there is brought about a change in the working point of the scavenging pump and/or the scavenging blower. The backwashing machines, designed on the basis of the resistance of unblocked nozzles, no longer provide the expected through-flow and therefore no good cleaning action of the backwashing medium. The quality of the filtrate is thereby negatively influenced. With the higher rinsing pressures that have, with time, come to be often used, numerous conventional filter nozzles fulfill their function only partially. In extreme cases the filter nozzles must from time to time be removed and cleaned at great cost. A known type of filter nozzle with superimposed discs, with spacer pins and horizontal slits, has brought an improvement. In this filter nozzle the flow takes place in one or the other direction through the spaces between the stacked discs. These spaces are larger than the slits themselves, so that due to the decrease in velocity a deposition of materials is possible. These materials are mostly particles which are whirled up during backwashing due to the higher velocity of the rinse water in the lower portion of the filter mass.
A filter nozzle described in Australian 52 363/64 is provided with annular filter discs. These filter discs are forced over a central pipe-shaped carrier. The filter discs are provided with projections in the region of their outer peripheries in order to leave free passage openings between two adjacent rings. The rings are further so formed that an outer periphery forms a radially inwardly widening passage space, as well as a separate inner passage space, likewise widening radially inwardly, and in this these passage spaces are separated from one another by an inlaid ring, which ring is intended to give the filter disc support an increased mechanical stability.
In order to be able to conduct water that enters the outer annular space radially into the inner annular space, both the filter discs and the intermediate ring are provided with openings parallel to the axis.
The inner annular chambers are connected, by means of longitudinal slits in the wall of the central pipe-shaped carrier, with the hollow interior of this carrier, so that the water that flows through the inner chamber ahead of it is conducted for radial flow through the outer chamber, then for flow parallel to the axis through the openings that are parallel to the axis, and finally radially into the pipe-shaped carrier. The smallest filtering openings for inflow of the water to be purified are the peripherally outer slits of the successive filter discs, while the through-flow cross sections enlarge towards the pipe-shaped carrier, which has the result that these filter nozzles present an extremely small pressure drop, but on the other hand exhibit the disadvantage that unfiltered finest particles can deposit in the interior upon the filter discs, particularly because of the retarded velocity of water through-flow. From this, as is noted in the description, there is an absolute requirement for cleaning of the filter interior from time to time by backwashing.
In another known construction (Great Britain, 2,037,601) there is described a filter nozzle with annular filter discs wherein suitable spacers are provided to make possible the admission of water between the filter discs for entry to the filter nozzle. In this the exclusion of the correspondingly small particles takes place on the outer surfaces of the filter discs. The inner chamber between the individual filter discs is enlarged by means of recesses in cross section, so that in this chamber there prevails a reduced stream velocity relative to the inflow velocity, which leads to a stoppage of the water between the discs as a result of the relatively narrow bore in the central portion, which has a central outlet opening with lateral bores. Through this, fine particles that are not filtered out seat themselves between the filter discs, which liquid backwashing of the filter guards against clogging.
In another construction (U.S. Pat. No. 4,267,045) there is described a filter nozzle with annular filter discs, in which nozzle likewise there is achieved a purely radial flow through the free space provided between the aggregated filter discs, radially towards the middle. With this, suitable elements provide for a constant change of direction, as in a maze, of the water that still contains the fine particles but is otherwise filtered. This has the disadvantage that this constant change of direction of the water running through it, with corresponding vortex generation and dead water zones, promotes a deposition of the fine particles, so that with this construction, too, a liquid backwashing is absolutely necessary to guard these spaces between the filter discs from blockages and precipitation out of the filter nozzle.
In a further prior publication (U.S. Pat. No. 3,784,015) there is provided a filter nozzle with filter discs that are arranged parallel to one another, which are so formed that between two successive discs a predetermined laminar flow of the water to be purified is assured, wherein centrifugal forces operate which deposit the particles in suitable pockets provided for them, and for the rest keep open the throughflow cross section for the water. A construction of this type requires, however, an emptying of the respective pockets, which is not possible in a simple manner. For this reason such a filter is not suitable for purification of large quantities of water.
The surfaces exposed to a deposit of solid materials, particularly the crosspieces of the filter rings, are dimensioned to be extremely small.